Role of phagotrophy in freshwater phytoplankton

Sammanfattning: Some phytoplankton have the ability to combine phototrophy and heterotrophy within the same individual, which is called mixotrophy. In this thesis, I adress mixotrophy and competitive interactions of mixotrophs and obligate auto- or heterotrophs in relation to the availability of nutrients, light and prey. In addition, I examine the biomass of mixotrophic phytoplankton and their grazing impact on bacteria in oligotrophic clear-water lakes. In nutrient enrichment experiments, mixotrophic phytoplankton were favored over their obligate autotrophic competitors at low availability of nutrients or light. Thus, the ingestion of prey may have provided the mixotrophs with the currently limiting nutrient (nitrogen, phosphorus, or both) as well as with carbon/energy when photosynthetic carbon acquisition was insufficient. In laboratory experiments with the mixotroph Poterioochromonas malhamensis, phototrophic growth supplemented heterotrophic growth when bacterial densities were similar to those found in oligo- to mesotrophic lakes. As a result, the mixotroph was able to dominate in terms of biomass over an obligate heterotroph (Spumella elongata) when light was available. Thus, depending on the prevailing environmental conditions, mixotrophy may be advantageous compared to obligate auto- or heterotrophy. In the examined oligotrophic clear-water lakes on the Faroe Island and in S. Sweden, mixotrophic phytoplankton composed a high proportion of the total algal biomass. Considering the high light availability in the epilimnia of the sampled lakes and the documented high photosynthetic capacities of the mixotrophic species present, bacterial ingestion was most likely a means to obtain limiting nutrients or growth factors. Although the grazing pressure exerted on bacteria by mixotrophs was low year round in Lake Skärlen (S. Sweden), the mixotrophs exerted a considerably higher grazing pressure on bacteria in the Faroese lakes. The observed differences may be a result of a higher nutrient competition in the Faroese lakes due to a higher importance of allochthonous carbon in these lakes.